1. Field of the Invention
The present invention relates to electrostatic toning apparatus of the type using a liquid toner and in particular to a toning apparatus which is provided with a toning member that has conductive and nonconductive elements thereon and which forms a laminar flow of toner liquid over the conductive element of the toning member.
2. Description of the Prior Art
In the color proofing industry latent images are typically formed on the surface of a photoconductive imaging member. The images are developed by the application of a liquid toner to the surface. The latent electrostatic image carried on the photoconductive surface may be envisioned as a collection of closely spaced pockets of electrostatic charge. The charges produce associated voltages on the surface of the member. The voltage magnitude determines the toner stack height and the image density in both highlight and shadow dot areas. However, when measured with an electrostatic voltmeter the highlight dots exhibit lower voltage than that of the shadow dots. The charge distribution for each of the edges of the highlight and shadow dots can be represented of a Gaussian distribution. The depiction in FIG. 1A shows a typical voltage representation in a highlight image dot H and a shadow dot S in a latent electrostatic image prior to toning.
It has been found that depending on what kind of half tone generation algorithm is used the average voltage on the smaller highlight image dot H is lower than that on the larger shadow image dot S. Thus, as noted earlier, in FIG. 1A the smaller voltage magnitude represents a latent image of a highlight dot H and the lager voltage magnitude represents a latent image of a shadow dot S.
Toning efficiency is a strong function of the dot voltage. The image quality of a proof is governed by the density of each individual color and each individual half tone dot as it is developed. To enhance the development of the latent image it has been found that the presence of a bias voltage during toning permits the larger shadow dot S to develop to completion faster than the smaller highlight image dot H. As a result, in the presence of a bias voltage during toning, the toner density of the finished shadow dot is substantially greater than that of the highlight dot. This situation is illustrated in FIG. 1B which depicts the toner density distributions for a developed highlight image dot H and a shadow image dot S when development occurs in the presence of a bias voltage.
It is conversely known that toning in a nonbiased environment permits the highlight image dot H to be developed rapidly. However, due to the strong fringe fields around the edges of the shadow image dot S, the latent shadow image dot S cannot be toned to full density in the same nonbias environment. FIG. 1C depicts the density distribution of toner when the highlight image dot H and the shadow image dot S have been toned in the absence of a bias voltage. The toner density distribution in the highlight image dot H is relatively uniform because the field distribution within the highlight image dot H is relatively uniform owing to the small dot size (see FIG. 1A). However, because of the fringe field the shadow image dot cannot be toned to a uniform density across the dot.
Accordingly, in view of the foregoing it is believed advantageous to provide a toning apparatus wherein the latent electrostatic images of both the highlight and the shadow image dots can each be toned to thier full density and to substantially equal densities.